 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / March 2006revving an engine to "warm it up"
I recently had a discussion on this topic. I have heard that revving the engine may do harm and does little to "warm" the car up. This is an older car with an engine that has been modified. This guy revvs the engine at the house and still seems to have trouble making it down his driveway (about 1/4 mile steep downhill) without stopping and revving his engine halfway down and then again at the bottom. Is this type of "warm up" accomplishing anything or is he just destroying his car? > I recently had a discussion on this topic. I have heard that revving > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 3 lines] > his engine halfway down and then again at the bottom. Is this type of > "warm up" accomplishing anything or is he just destroying his car? It's probably not good for the car; however, if the engine is "modified" it may have issues with a cold start, esp. if he has a chokeless carburetor (sometimes people who don't live in particularly cold climates will mill the choke housing off the carb for better airflow) therefore he may have to play with it a little to keep it from stalling. I know a friend of mine has an old Studebaker with a 383 Chev "crate motor" in it; he doesn't have the specs on the motor (bought the car pretty much as it is) but it is pretty hairy (and fun to drive) even after both of us threw all our tricks at it to get it to start and run without a long period of sitting still and revving the engine, the "fast idle" had to be set at about 2000 RPM and it still doesn't always start on the first crank. (of course, it also has dual AFBs on a low rise Offy intake, probably not the hot setup for anything but WOT but it looks cool) I am guessing the PO didn't ever plan on driving the car in cold weather; neither carb had a choke on it when he got it. We did install an electric choke on the rear carb and that did make a lot of difference, although it's still not the kind of car you'd want to hop in and drive to work (well, I might, but I'd still probably have to leave it sit and "idle" - using the term loosely - for a minute or two in the AM before driving off.) But yes, your basic premise is correct - the best way to handle a normal cold start, unless it's bitterly cold, is to simply start the car and drive off, but drive "easy" until the engine is fully up to operating temperature. nate  Signaturereplace "fly" with "com" to reply. http://home.comcast.net/~njnagel > I recently had a discussion on this topic. I have heard that revving > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 3 lines] > his engine halfway down and then again at the bottom. Is this type of > "warm up" accomplishing anything or is he just destroying his car? Both! Yes, that can reduce the time to warm the engine. But, WHY are we warming the engine? Primarily to ensure proper lubrication from the oil. If we rev it up before that, we are not getting proper lubrication, so why do it? This is kind of self defeating. We ordinarily think of warming the engine before stressing it as contributing to reduced wear. But if we wear it excessively WHILE warming, we gain nothing. Admittedly, if this is an older car with a carburetor, drivability improves when the car is warmed up. But again, drivability is bad when he is revving it anyway. Better to ease it along and warm it that way. If it tends to stall, he should play with fast idle rpm setting. Not the best for it, but likely it is stalling out so needed. I have a carb engine that came stock with a 'timer' choke. The sucker goes open according to how long the engine has been running with no reference to how 'hot' the engine actually is. These types of chokes are a pain in the a.s up here in Canada. I would stall every time I let off the gas pedal in the first 1/4- 1/2 mile unless I revved it until the engine had really warmed up. 3 foot driving style or heel toe on the gas brake for a while. I fixed it's case by putting a manual choke in. I now can keep the fast idle at 1200 until it is actually warmed up and drive away in any temperature. Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) > I recently had a discussion on this topic. I have heard that revving > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 3 lines] > his engine halfway down and then again at the bottom. Is this type of > "warm up" accomplishing anything or is he just destroying his car? > I recently had a discussion on this topic. I have heard that revving > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 3 lines] > his engine halfway down and then again at the bottom. Is this type of > "warm up" accomplishing anything or is he just destroying his car? Exhaust is often shut off, and other things are closed for quick warmup. shouldn't push the engine under these conditions as you can cause stuff to wear out more quickly. Probably not as hard on newer cars, but also even less beneficial. SignatureThank you, "Then said I, Wisdom [is] better than strength: nevertheless the poor man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16 >> I recently had a discussion on this topic. I have heard that revving >> the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 3 lines] >> his engine halfway down and then again at the bottom. Is this type of >> "warm up" accomplishing anything or is he just destroying his car? It depends on the car. If its an older car that is unmodified or lightly modified, he's got a PROBLEM that needs fixing (choke flooding the engine or not applying sufficiently, etc.) Even fairly heavily modified engines can be made to behave 'tolerably well' when cold if all the systems are working right. If its an extremely highly modified car, being horribly cold-natured is just part and parcel of living with a high-strung engine. Its hard on it, but its probably not designed with longevity anyway. If its a question of stopping to rev up and blow the unburned fuel out, versus stalling and restarting over and over- then revving it a bit is the only choice. > Exhaust is often shut off, That is without a doubt one of the sillier things I've ever read on an automotive tech newsgroup. The BIGGEST risks for a cold engine are: 1) fuel dilution in the engine oil and "washing" down the cylinder walls with raw fuel. Mostly a problem with carb'd cars, though EFI does this to s limited degree as well. Prolonged idling to warm the car makes this much worse than just driving gently. 2) Excessive wear because clearances are larger when cold. This isn't normally a big deal as long as you give it time to get the oil circulating before putting a full load on it. Even audible piston slap doesn't really cause excessive wear most of the time. BUT, the car is more sensitive to how HARD its driven when cold. Romping on a stone-cold engine will wear it more than driving it gently for a few minutes until things come up closer to operating temperature. > > I recently had a discussion on this topic. I have heard that revving > > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 5 lines] > > Exhaust is often shut off, and other things are closed for quick warmup. Do you mean that incoming air is first routed over the exhaust manfold to warm it and thereby speed engine warmup? > shouldn't push the engine under these conditions as you can cause > stuff to wear out more quickly. Probably not as hard on newer cars, but [quoted text clipped - 5 lines] > "Then said I, Wisdom [is] better than strength: nevertheless the poor > man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16 >> > I recently had a discussion on this topic. I have heard that revving >> > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 3 lines] >> > his engine halfway down and then again at the bottom. Is this type of >> > "warm up" accomplishing anything or is he just destroying his car? ============================================================ >> Exhaust is often shut off, and other things are closed for quick warmup. What really happens in a *modern car* is the thermostat opens gradually, and in cold weather might take 20 minutes to open fully. It doesn't *snap open* like a switch. It starts out fully closed, but as soon as the engine gets to about 170 degrees F, it opens a little, and as the coolant becomes hotter, it opens more, until at around 200F it is fully open and oscillates back and forth allowing the water to become a little warmer, and than a little cooler, producing something like a sine wave with respect to the temperature signal coming from the engine heat sensor. >Do you mean that incoming air is first routed over the exhaust manfold >to warm it and thereby speed engine warmup? I don't see how that would make any sense. True that the coolant is circulated through the heater core according to dashboard settings, rather than the thermostat. Lg >> shouldn't push the engine under these conditions as you can cause >> stuff to wear out more quickly. Probably not as hard on newer cars, but [quoted text clipped - 5 lines] >> "Then said I, Wisdom [is] better than strength: nevertheless the poor >> man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16 > >> > I recently had a discussion on this topic. I have heard that revving > >> > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 25 lines] > > Lg Go back and look at cars with carbs in the '70s and you will see this air ducking from the exhaust manifold to the air cleaner. You talk about "What really happens in a *modern car*" well computers and EFI are a different world from a modified engine with not exhaust heat to the intake manifold, no choke on the 2 4 bbl carbs and that has a cam that when the engine is warmed and running normal will not idle at less than 1200 rpm and best of all was high test leaded gas for 36 cents a gallon. Stan > >> shouldn't push the engine under these conditions as you can cause > >> stuff to wear out more quickly. Probably not as hard on newer cars, but [quoted text clipped - 5 lines] > >> "Then said I, Wisdom [is] better than strength: nevertheless the poor > >> man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16 >> >> > I recently had a discussion on this topic. I have heard that revving >> >> > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 25 lines] >> >> Lg ============================================================== >Go back and look at cars with carbs in the '70s and you will see this >air ducking from the exhaust manifold to the air cleaner. I believe it. If you're into antique automobile, there are better n/g's than this one. > You talk about >"What really happens in a *modern car*" Yes, that's my point. Modern car. > well computers and EFI are a >different world from a modified engine with not exhaust heat to the >intake manifold, no choke on the 2 4 bbl carbs and that has a cam that >when the engine is warmed and running normal will not idle at less than >1200 rpm What a waste of fuel. My engine idles at 700 rpm. > and best of all was high test leaded gas for 36 cents a gallon. >Stan There was nothing *best* about those old tanks. Lg > >> >> > I recently had a discussion on this topic. I have heard that revving > >> >> > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 51 lines] > > Lg > >> >> > This is an older car with an engine that has been modified. When someone asks about a modified engine telling them about your factory stock / modern car does them little good. The point is to answer the question that was ask. Stan >> >> >> > I recently had a discussion on this topic. I have heard that revving >> >> >> > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 58 lines] >the question that was ask. >Stan Hair dryer. Is what I use on my landscaping equipment in wintertime to get engines/carburetors warm enough to start. It is the only thing that works for me 100 times out of 100. No iced carbs, no frozen fuel lines, just vrooom putt putt putt Hair dryer. For my landscaping equipment. A car? Lift the hood, use remote switch, blast hairdryer on carb. vrooom putt putt putt I still say, 1.8 liter is little more than a lawnmower engine where I live. Not to sound like a snob, I owned a Renault R8, and a Fiat Sardine Can for many years. I know the problems that go along with these *more economical* vehicles. That's why I will never own one again. Lg > >> >> >> > I recently had a discussion on this topic. I have heard that revving > >> >> >> > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 75 lines] > > Lg You must be use Obfuscate on your replies. Stan > >> > I recently had a discussion on this topic. I have heard that revving > >> > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 16 lines] > like a sine wave with respect to the temperature signal coming from > the engine heat sensor. That's interesting I suppose, but it has absolutely nothing to do with the O.P.'s comment. > >Do you mean that incoming air is first routed over the exhaust manfold > >to warm it and thereby speed engine warmup? > > I don't see how that would make any sense. True that the coolant is > circulated through the heater core according to dashboard settings, > rather than the thermostat. That setup was very common on cars up through the late 80's and early 90's. A shroud surrouds the intake manifold and an aluminum air intake hose connect the shroud to the front of the air cleaner. There is a temperature controlled air valve is used to direct air. Do a little research before making such a flat statement. > Lg > [quoted text clipped - 7 lines] > >> "Then said I, Wisdom [is] better than strength: nevertheless the poor > >> man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16 > That setup was very common on cars up through the late 80's and early > 90's. A shroud surrouds the intake manifold and an aluminum air intake > hose connect the shroud to the front of the air cleaner. There is a > temperature controlled air valve is used to direct air. Do a little > research before making such a flat statement. Exhaust manifold. >> >> > I recently had a discussion on this topic. I have heard that revving >> >> > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 26 lines] >> circulated through the heater core according to dashboard settings, >> rather than the thermostat. ============================================================== >That setup was very common on cars up through the late 80's and early >90's. A shroud surrouds the intake manifold and an aluminum air intake >hose connect the shroud to the front of the air cleaner. There is a >temperature controlled air valve is used to direct air. Do a little >research before making such a flat statement. Maybe on -some- makes of vehicles...certainly nothing I ever drove. IIRC, in the 80's and 90's I had a lot of *vintage* automobiles...nothing I paid over $200 for. The one thing they had in common was amazing horsepower and an insatiable appetite for fuel. You could watch either the speedometer or the fuel gauge...sometimes it was hard to tell which was which. I might still be driving something similar today if it weren't for the strict emissions tests run by the State of Illinois. Those vehicles wouldn't/couldn't Pass no matter what you did to them. I did drive a 442 one summer, and watched that gas gauge drop like a barometer in a tornado. You could EASILY go through 1/4 tank of fuel in 10 minutes. An eighth of a tank in one acceleration ( that would glue you to the drivers seat...or rather IMBED you in it ). here is something to refresh your memory: ------------------------------------------------------------------------------------------------- 400 cubic inches, 4-barrel carburetor, 2 exhausts ------------------------------------------------------------------------------------------------- That was a jet fighter. All it was missing was wings. 345 horsepower. Compare that with the dinky sh.t on the roads today. They will not build another 442 in our lifetimes, I'm sad to say. Fuel has become too expensive for such a beast. At the opposite end of the spectrum is the limp-wristed Eurocar, coming to a neighborhood near you soon. Do I care about yesterday's vehicles? Only if I'm driving one. Not too many are. Show me a 442 on the road today...ANYWHERE. Lg >> Lg >> [quoted text clipped - 7 lines] >> >> "Then said I, Wisdom [is] better than strength: nevertheless the poor >> >> man's wisdom [is] despised, and his words are not heard." Ecclesiastes 9:16 > > > I recently had a discussion on this topic. I have heard that revving > > > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 8 lines] > Do you mean that incoming air is first routed over the exhaust manfold > to warm it and thereby speed engine warmup? He is talking about a heat riser valve that shuts off one bank of a V engine's exhaust and routes it across the intake for warming the fuel/air mix. Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) > > > > I recently had a discussion on this topic. I have heard that revving > > > > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 19 lines] > Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 > (More Off Road album links at bottom of the view page) I believe this is what he is talking about. Scan from an old Chilton's. http://users.erols.com/srweiss/expipe.gif Stan > > > > > I recently had a discussion on this topic. I have heard that revving > > > > > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 23 lines] > http://users.erols.com/srweiss/expipe.gif > Stan No, all carb engines have heated air cleaners, that has zip to do with the exhaust except to suck warm air from a box over it. Modified engines tend to have aftermarket air cleaners missing this so can ice up easy. That heat pipe opens and closes as the temp of the air filter changes so you can drive without the carb turning into a block of ice. They also need to heat the air in the manifold for a more efficient warm up. This is done various ways. Some older Jeeps for instance used a heat riser like the V8's but the newer carb'ed Jeeps used a heater element at the carb base with engine coolant flowing through the manifold. The older V6's and V8 used a manifold cross over and shut off one bank of the exhaust with a weighted flap on a bi-metal spring. I must be showing my age here..... Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) > > > > > > I recently had a discussion on this topic. I have heard that revving > > > > > > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 40 lines] > > I must be showing my age here..... I don't think so, if you say all all carb engines have heated air cleaners. In the '60s check out the muscle cars and show me a heated air cleaner. Stan > Mike > 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 > 88 Cherokee 235 BFG AT's > Canadian Off Road Trips Photos: Non members can still view! > Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 > (More Off Road album links at bottom of the view page) > > > > > > > I recently had a discussion on this topic. I have heard that revving > > > > > > > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 45 lines] > cleaner. > Stan Ya ya, they fall into the modified class in my book and I have helped a pile of those owners retrofit the heat pipes once they realized their carbs were turning into ice blocks come winter so they could use them year round. Along with a second set of rims and snow tires.... Most just parked them come cold weather. > > Mike > > 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 > > 88 Cherokee 235 BFG AT's > > Canadian Off Road Trips Photos: Non members can still view! > > Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 > > (More Off Road album links at bottom of the view page) > > > > > > > > I recently had a discussion on this topic. I have heard that revving > > > > > > > > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 51 lines] > year round. Along with a second set of rims and snow tires.... Most > just parked them come cold weather. Without knowing what tempertures you see in the winter I cannot say. I know here in Philadelphia PA I drove my '67 GTO all year round in the late '60s as it was my only car. The only problem could be some water in the gas that would want to freeze and some dry gas toke care of that. Stan > > > Mike > > > 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 > > > 88 Cherokee 235 BFG AT's > > > Canadian Off Road Trips Photos: Non members can still view! > > > Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 > > > (More Off Road album links at bottom of the view page) its not good to "rev" a cold engine. the heat "riser" routes exhaust heat....off the manifolds up to the air filter housing. Its not exhaust...its heat. There is a valve in the air filter housing that controls the heat (it often wears out over time. THe valve regulates the amount of cold air through the regular air pathway, and the amount of hot air coming off the exhaust. Thats the way it works on the older GM vehicles. I don't imagine that any car re-routes exhaust....its just the heat coming off the manifolds. No connection to turb equipped cars and trucks. >> > > I recently had a discussion on this topic. I have heard that revving >> > > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 19 lines] >Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 >(More Off Road album links at bottom of the view page) ----------- Elbert ask@me.com > its not good to "rev" a cold engine. > > the heat "riser" routes exhaust heat....off the manifolds up to the > air filter housing. Heat riser or more correctly, EFE (early fuel evaporation) is often a vacuum or bi-metal controlled flap in one of the exhaust manifolds that re-directs hot exhaust gas to a chamber on the bottom side of the intake manifold underneath the carburetor. This passage often serves as the feed to the EGR valve also. Another method is to use an electrically heated grid that mounts under the carburetor, i.e., early 80s GM "X" and FWD "A" bodies. > Its not exhaust...its heat. It's both. > There is a valve in > the air filter housing that controls the heat (it often wears out over > time. THe valve regulates the amount of cold air through the regular > air pathway, and the amount of hot air coming off the exhaust. Thats > the way it works on the older GM vehicles. You're thinking of the Thermostatic Air Cleaner (Thermac). Entirely possible to have both a heat riser -and- a Thermac. > I don't imagine that any > car re-routes exhaust.... Here's an example of bi-metal spring operated one: http://cgi.ebay.com/ebaymotors/Scout-800B-304-V-8-Heat-riser-valve _W0QQitemZ8042253230QQcategoryZ33635QQrdZ1QQcmdZViewItem Here's an example of a vacuum operated one: http://cgi.ebay.com/ebaymotors/83-88-Monte-Carlo-SS-Heat-Riser-Exh aust-Spacer-PERFECT_W0QQitemZ8043114547QQcategoryZ33635QQrdZ1QQcmd ZViewItem > its just the heat coming off the manifolds. That would be the thermostatic air cleaner. >> its not good to "rev" a cold engine. >> [quoted text clipped - 37 lines] > >That would be the thermostatic air cleaner. now that I think about it...I believe you are right. Many of the 70's year model cars had all kind of crap in the emission controls, and other junk. I think I do remember some type of valve in the exhaust manifold (one side) of my 78 z/28. Of course those went in the garbage (had to install headers). I believe it was a "butterfly" value in the exhaust manifold and had metal rod that operated it off a vaccum valve (is this right....anyone recall the GM Crap?) I bought the car used and the first thing that went was that valve "thing" in the exhaust, and on went dual exhaust. Then the intake and the smog carb went away too. Dual plane and holley went on next. I was wrong in my statement above... thats been a few years ago. I know that "valve thing" does not do much justice to the item but maybe you'll understand :) Thats been 20+ years ago now. Boy all that crap on those cars was about useless. I remember the smog pumps, and all the crazy vaccum lines. I think the stock 78 z/28 had about 180 HP...boy what a machine :) The factory air clearner went out the door too. I had a lot of fun with that car....especially after it ate a rod bearing....re-built the motor / trans and added a mild stall converter. Pretty hot street machine then. Long story short....I was wrong. ----------- Elbert ask@me.com Nope, you are talking about the thermostatically controlled air filter and it's heat pipe. The GM V8's and V6's of the 70's and 80's with carbs and TBI's used a heat riser valve to shut off the driver's side exhaust manifold forcing all the exhaust to cross over under the intake manifold and exit out the passenger side pipe until it warmed up. I haven't checked the newer ones specifically for that. Here is a link that shows both: http://www.hedman.com/pages/emissions-access.html The first photo shows the air filter heat pipe setup and the last photo shows the heat riser valve to shut off the exhaust on that pipe. Mike > its not good to "rev" a cold engine. > [quoted text clipped - 33 lines] > Elbert > ask@me.com > Nope, you are talking about the thermostatically controlled air filter > and it's heat pipe. [quoted text clipped - 13 lines] > > Mike Again here is the link which shows a GM engine from the '70s with exactly what Neil is talking about. http://users.erols.com/srweiss/expipe.gif Stan > > its not good to "rev" a cold engine. > > [quoted text clipped - 33 lines] > > Elbert > > ask@me.com you are right.... I got caught up on the heat pipe to the carb and I clearly forgot about the other stuff. I spent a lot of time removing all that junk. It did not stay on the car long (the one I had at the time) back in my HS days. (early 80's) Thanks for correcting me....I should have thought through my answer and done a little research prior to my post. Elbert >Nope, you are talking about the thermostatically controlled air filter >and it's heat pipe. [quoted text clipped - 51 lines] >> Elbert >> ask@me.com ----------- Elbert ask@me.com thanks for the link.... yea I clearly forgot all about that... I remember telling the exhaust guy to through that crap away and put on some real dual exhaust. You guys were right on. >Nope, you are talking about the thermostatically controlled air filter >and it's heat pipe. [quoted text clipped - 51 lines] >> Elbert >> ask@me.com ----------- Elbert ask@me.com >>>Exhaust is often shut off, and other things are closed for quick warmup. >> [quoted text clipped - 4 lines] > engine's exhaust and routes it across the intake for warming the > fuel/air mix. Which is absolutely NOT the same as the exhaust being "shut off." The heat riser is spring loaded so that if you do get on the throttle while its still "closed" the exhaust pressure will swingit open and let the exhaust take its normal route. > > > I recently had a discussion on this topic. I have heard that revving > > > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 8 lines] > Do you mean that incoming air is first routed over the exhaust manfold > to warm it and thereby speed engine warmup? Two things can happen - 1) a "heat riser valve" will shut and direct exhaust away from its normal path of straight from the valves through the exhaust manifold(s) and out the exhaust pipe(s) to an auxiliary passage under the carburetor, to help the carburetor get up to operating temperature quickly. This valve will have a thermostatic bimetal spring to open it again when the appropriate temperature is reached. (something to check when a car has been sitting a while; this valve will often be rusted in the shut position causing vapor lock problems when warm) 2) a valve will close in the snorkel of the air cleaner causing the carb to suck its intake air not straight through the snorkel but from a little sheet metal "heat stove" strapped or bolted to an exhaust manifold, also to help the carburetor warm up quicker. Again, when warm, this valve will open to prevent the carb from overheating. On a "modified" engine, it's entirely likely that some of the first modifications made were to replace the manifolds with tubular headers and the original air cleaner with some freer-flowing aftermarket unit, which will provide for better airflow and thus more efficiency and power under normal operating conditions, but will also eliminate both of the functions described above and tend to make the car a little "cold blooded" unless it's operated only in a temperate climate. This can be compensated for somewhat by adjusting the choke richer, but at the expense of economy until the engine is fully warm (and it will still prolong the period of time that the carburetor is operating in "warm up" or choked mode) As I posted before, sometimes people with a really "hot" car will mill off the choke horn in the interest of improved airflow; when combined with the other mods mentioned above, this will make the car all but impossible to drive without revving the snot out of it to keep it lit until it reaches operating temperature. I don't know how old "older" is, but any car I've ever worked on has at least a choke and the heat riser valve. I think the heat stove/snorkel valve was introduced in the late 60's or early 70's (depending on manufacturer.) nate > As I posted before, sometimes people with a really "hot" car will mill > off the choke horn in the interest of improved airflow; when combined > with the other mods mentioned above, this will make the car all but > impossible to drive without revving the snot out of it to keep it lit > until it reaches operating temperature. And if the cam is radical enough (lots of overlap) then even using the choke won't do much good since so little vacuum is developed in the intake manifold. That's really the reason most people eliminate the choke at some point- it stops being useful and is *just* in the way of air flow. >And if the cam is radical enough (lots of overlap) then even using the >choke won't do much good since so little vacuum is developed in the >intake manifold. That's really the reason most people eliminate the >choke at some point- it stops being useful and is *just* in the way of >air flow. Then they're those carbs without choke butterflies to begin with, but enrichment jets instead, like Strombergs and certain Motorcraft (Ford) models. Pulling out the "Choke" lever on an old Volvo just slides a metering rod out of the auxiliary jet in each carb. :-) Signature-john wide-open at throttle dot info > I recently had a discussion on this topic. I have heard that revving > the engine may do harm and does little to "warm" the car up. This is an [quoted text clipped - 3 lines] > his engine halfway down and then again at the bottom. Is this type of > "warm up" accomplishing anything or is he just destroying his car? He will minimize the wear on the engine during warmup by just getting in the car and driving it, keeping the engine at a steady and low rpm. If the engine doesn't idle well when cold it is probably in need of repair. Much less likely in my experience is that the car is a highly tuned street racer. The many cars I've driven over the past 15 years all use an engine computer to make warmup and overall running very smooth. If taken care of there should be no problem with a modern engine holding an idle when cold. |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.